1. Field of the Invention
The present invention relates to a process for the manufacture of a soybean curd (so-called "Tofu" in Japan) and more particularly to a process for commercially manufacturing a packed aseptic soybean curd of high quality, in a reasonable manner.
2. Related Arts
The soybean curd has an excellent flavour and mouth-feeling and can be said to be one of the high nutrient vegetable foods. Further, the soybean curd has a subtle taste and white color tone and thus shows an adaptability to various cooking products.
Hitherto, various processes have been proposed to manufacture the soybean curd. Typical conventional processes therefor will be explained with reference to a flow-chart shown in FIG. 4.
The conventional processes may be classified into (A) fine textured soybean curd (so-called "Kinu tofu" in Japan), (B) rough textured soybean curd (so-called "Momen tofu" in Japan), (C) packed soybean curd and (D) packed aseptic soybean curd. All of the conventional processes are common in steps of well-selecting and washing the soybeans, soaking the soybeans in water for 8 to 15 hours, adding water and a defoamant, grinding the soybeans to form a green slurry (or Go-puree which is called in Japan as "Namago"), directly heating the green slurry or blowing water-steam therein to form a thermally treated slurry (or cooked slurry which is called in Japan as "Go"), and squeezing the cooked slurry to obtain a soybean milk by separating a curd residue (so-called in Japan as "Okara").
According to one of the classic processes, a coagulant is added in a soybean milk and the mixture is poured into a mold to leave as it is to cause a coagulation thereof, and then exposed to water to obtain the fine textured soybean crd (A). According to another classic process, a coagulant is added in a soybean milk to allow a sedientation of coagulated substance, some amount of a spernatant is skimmed (this step is called in Japan a "Yutori") and the resulting precipitated substance is scooped out, charged into a mold, pressed therein, and exposed to water to obtain the rough textured soybean curd (B). In these classic processes, a bittern was formerly employed as the coagulant but recently, a coagulant comprising calcium sulfate as main component has been employed, since a coagulation reaction can be delayed and a resulting soybean curd shows a good water-holding capacity and somewhat more smooth feeling in mouth.
On the other hand, the production of the so-called packed soybean curd (C) has been made possible with gluconodeltalactone (hereinafter referred to as "GDL") which was developed as a new type coagulant for the soybean curd, after the second world war. The soybean curd of this kind is manufactured by cooling and deaerating a soybean milk, adding GDL therein and mixing same, filling in a container, and heating to cause a coagulation. According to this conventional process, the thermal coagulation is carried out after filling the soybean milk into the container, which makes an automatic and continuous mass production of the soybean curd easy, so that a share of the soybean curds manufactured by this type process gradually prevails in the market.
Finally, the so-called packed aseptic soybean curd (D) has been developed to provide a soybean curd which can be stored for a long period of time, at room temperature. The conventional packed aseptic soybean curd has been manufactured by thermally treating a soybean milk to sterilize the same, cooling and deaerating the sterilized soybean milk, adding thereto and mixing same with a sterilized GDL solution, filling the mixture into a sterilized container under an aseptic atmosphere, sealing the container under an aseptic condition, and heating the mixture to cause a coagulation thereof see U.S. Pat. No. 4,000,326 and U.S. Pat. No. 4,514,433.
The fine textured and rough textured soybean curds to be obtained by said classic processes show drawbacks of that its storage stability is quite low; that is perishable, even if stored under a lower temperature condition, the soybean curd tends to break-down easily, during a transportation thereof, and its manufacture requires a skill, which makes a large scale production difficult.
While, the conventional process for manufacturing the packed soybean curds somewhat dissolves the drawbacks encountered in such classic processes, but the resulting soybean curd is to be marketed and stored at a low temperature and its storage stability can not be made so high. In order to eliminate the disadvantages inherent to the packed soybean curd, the process for manufacturing the packed aseptic soybean curds has been proposed, but the soybean curd manufactured by proposed this type processes is not sufficient in that it is poor in mouth-feel and flavour, and shows a syneresis.